Guide
What is the heat index?
Updated 2026-07-10
Quick answer
The heat index is the National Weather Service’s calculated “how hot it feels” temperature. It combines air temperature and relative humidity — because humidity slows how efficiently sweat evaporates — into a single number the NWS assigns to a risk band (Caution, Extreme Caution, Danger, Extreme Danger).
Why temperature alone doesn’t tell you how hot it feels
Your body cools itself mainly by evaporating sweat. When relative humidity is high, sweat evaporates more slowly, so your body sheds heat less effectively — the same air temperature can feel noticeably hotter on a humid day than a dry one. The heat index folds that effect into one number so “how hot it feels” is comparable across different humidity conditions.
The Rothfusz regression, in plain words
The NWS starts with a simpler Steadman-based estimate, then — once that estimate reaches roughly 80°F — switches to a more precise formula published by Lans Rothfusz: an eight-term regression built from air temperature and relative humidity. Two further corrections apply at the edges: a small subtraction in low humidity (below 13%) and a small addition in high humidity (above 85%) within specific temperature ranges. The published formula and its exact coefficients are on the NWS/WPC heat index page.
The NWS risk bands
The NWS groups heat index values into four bands: Caution (80–90°F), Extreme Caution (90–103°F), Danger (103–125°F), and Extreme Danger (125°F and above). Below 80°F there is no assigned band. These bands describe general population risk from prolonged exposure and physical activity — they are published guidance, not a guarantee about any individual.
Where the numbers come from
The heat index formula and its validity range are published by NOAA’s Weather Prediction Center. Swelter implements that published formula exactly and validates its output against the published NWS heat-index chart — it does not modify or approximate the math.
Not affiliated with, endorsed by, or connected to OSHA, NIOSH, CDC, or the National Weather Service. Threshold values and guidance are based on their published public information. Swelter is an informational tool, not a medical device. It does not diagnose or prevent heat illness.